Fiction narrative is a river of cause and effect which sweeps the reader and the characters through the novel. What happens in each scene affects what happens through the rest of the novel, and main characters should change as these events affect them.
If the sweet heroine has to kill someone to save her lover’s life, that death should change her, and that person’s death should affect the events of the novel.
If that death scene has no effect on either the heroine or the plot, it is a bubble scene. The reader may also decide that she’s not so sweet and may be a psychopath.
If she nearly makes love to another man and doesn’t think about her true love and that event does nothing to change her or the plot, that’s a bubble scene. You’ve also changed the reader’s view on your heroine’s worthiness for a happily ever after.
Bubble scenes are emotional failures because the reader loses their connection to the story you want to tell. These scenes also change the reader’s perception of your character.
If a scene has nothing to do with the rest of the novel, you should ask yourself if it should be included. When the answer is no, that bubble scene should be popped.